Dry hemostatic compositions and methods for their preparation

ABSTRACT

Dry cross-linked gelatin compositions are prepared that rapidly re-hydrate to produce gelatin hydrogels suitable as hemostatic sealants. Gelatin is cross-linked in the presence of certain re-hydration aids, such as polyethylene glycol, polyvinylprovidone, and dextran, in order to produce a dry cross-linked gelatin powder. The use of the re-hydration aids has been found to substantially increase the re-hydration rate in the presence of an aqueous re-hydration medium, typically thrombin-containing saline.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to collagen and collagen-derivedcompositions and methods for their preparation. In particular, thepresent invention relates to a method for producing a dry cross-linkedgelatin or other collagen or collagen-derived composition which iscapable of absorbing water at an enhanced rate.

Fusion Medical Technologies, Inc., assignee of the present application,produces a hemostatic composition under the FloSeal® trade name. TheFloSeal® product is available in a package including two syringes. Afirst syringe is filled with granules of cross-linked bovine gelatinwhich are pre-hydrated with a buffer solution. The gelatin hydrogelcontains about 85% (w/w) water and is in the form of a flowablehydrogel. Immediately prior to use in the operating room, thrombin inaqueous saline is mixed with the gelatin hydrogel. The thrombin isprepared in saline and drawn up in a second syringe, and the syringesare connected together permitting mixing of thrombin and the gelatin.

The resulting mixture of the gelatin hydrogel granules and the thrombinhas been found to be a highly effective hemostatic sealant when appliedto a bleeding site. Typically, the sealant will be applied through thesyringe in which it has been mixed to the bleeding site. Blood willpercolate through the resulting bed of hydrogel granules, and thethrombin reacts with fibrinogen in the blood to form a fibrin clotaround the gelatin to seal the bleeding site.

Although highly effective, the present FloSeal® product has a limitedshelf life. It is believed that the stability of the gelatin is reducedby hydrolysis of the packaged hydrogel. To limit possible hydrolyticdegradation, the FloSeale® product is usually shipped in atemperature-protected packaging.

For these reasons, it would be desirable to provide improved hemostaticsealing compositions of the type which combine a collagen, gelatin, orother collagen-derived hydrogel with a thrombin-containing aqueoussolution. In particular, it would be desirable to provide suchcompositions in a form which would be resistant to hydrolyticdegradation and which would therefore have a longer shelf life. It wouldbe particularly desirable to provide improved compositions having bothcomparable hemostatic activity to the present FloSeal® product andlonger shelf lives. Such compositions would be most beneficial if theycould be rapidly re-hydrated for subsequent use, typically so that theycould be extruded through a syringe. At least some of these objectiveswill be met by the inventions described hereinafter.

2. Description of the Background Art

The FloSeal® product available from Fusion Medical Technologies, Inc.,is described in Hood et al., Efficacy of Topical Hemostat FloSeal™ inVascular Surgery, an Abstract funded by Fusion Medical Technologies,Inc., which was publicly presented in September 1999. Patents coveringthe FloSeal® product include U.S. Pat. Nos. 6,063,061 and 6,066,325. Adual syringe system suitable for mixing and delivering a collagen,gelatin, or other collagen-derived component and a thrombin component ofthe FloSeal™ product is described in U.S. Pat. No. 5,908,054. Thecomplete disclosures of each of these patent references is herebyincorporated by reference.

BRIEF SUMMARY OF THE INVENTION

The present invention provides improved hemostatic sealing compositions,methods for preparing such improved compositions, and kits comprisingthe improved compositions. The methods and compositions will beparticularly useful for providing hemostasis at bleeding sites,including surgical bleeding sites, traumatic bleeding sites and thelike. An exemplary use of the compositions may be in sealing the tissuetract above a blood vessel penetration created for vascularcatheterization.

The compositions comprise a dry cross-linked gelatin powder which hasbeen prepared to re-hydrate rapidly. The gelatin powder preferablycomprises relatively large particles, also referred to as fragments orsub-units, as described in U.S. Pat. Nos. 6,063,061 and 6,066,325, thefull disclosures of which have previously been incorporated byreference. A preferred particle size will be the range from 150 μm to750 μm, but particle sizes outside of this preferred range may find usein many circumstances. The dry compositions will also display asignificant “equilibrium swell” when exposed to an aqueous re-hydratingmedium. Preferably, the swell will be in the range from 400% to 1000%,but may fall outside of this range as set forth in the above-referencedpatents. “Equilibrium swell” may be determined by subtracting the dryweight of the gelatin hydrogel powder from its weight when fullyhydrated and thus fully swelled. The difference is then divided by thedry weight and multiplied by 100 to give the measure of swelling. Thedry weight should be measured after exposure of the material to anelevated temperature for a time sufficient to remove substantially allresidual moisture, e.g., two hours at 120° C. The equilibrium hydrationof the material can be achieved by immersing the dry material in asuitable re-hydrating medium, such as aqueous saline, for a time periodsufficient for the water content to become constant, typically for from18 to 24 hours at room temperature.

The dry cross-linked gelatin powders of present invention will usuallyhave some residual moisture, but will be sufficiently dry to achieve thedesired stability and extended shelf life. Typically, the drycompositions will have a moisture content below 20% by weight (w/w) orless, preferably having a moisture content in the range from 5% byweight to 15% by weight. To maintain dryness, the compositions willtypically be packaged in a manner suitable to prevent moistureincursion, as described in more detail in connection with the kits ofthe present invention.

In one particular aspect of the present invention, compositions willcomprise cross-linked gelatin powders having a moisture content of 20%(w/w) or less, wherein the powder was cross-linked in the presence of are-hydration aid so that the powder has an aqueous re-hydration ratewhich is at least 5% higher than the re-hydration rate of a similarpowder prepared without the re-hydration aid. The “re-hydration rate” isdefined herein to mean the quantity of an aqueous solution, typically0.9% (w/w) saline, that is absorbed by a gram of the powder (dry weightbasis) within thirty seconds, expressed as gm/gm. Particular techniquesfor measuring this rate are described in the Experimental sectionhereinafter. Preferred compositions of the present invention will have are-hydration rate of at least 3 gm/gm, preferably at least 3.5 gm/gm,and often 3.75 gm/gm or higher. Re-hydration rates of similar powdersprepared without the re-hydration aids are typically below three, and apercentage increase in re-hydration rate will usually be at least 5%,preferably being at least 10%, and more preferably being at least 25% orhigher.

The dry cross-linked gelatin powders of the present invention havingimproved re-hydration rates are preferably obtained by preparing thepowders in the presence of certain re-hydration aids. Such re-hydrationaids will be present during the preparation of the powders, but willusually be removed from the final products. For example, re-hydrationaids which are present at about 20% of the total solids content, willtypically be reduced to below 1% in the final product, often below 0.5%by weight. Exemplary re-hydration aids include polyethylene glycol(PEG), preferably having a molecular weight of about 1000;polyvinylpyrrolidone (PVP), preferably having an average molecularweight of about 50,000; and dextran, typically having an averagemolecular weight of about 40,000. It is preferred to employ at least twoof these re-hydration aids when preparing the compositions of thepresent invention, and more particularly preferred to employ all three.

The methods of the present invention thus comprise providing an aqueoussolution of a non-cross-linked gelatin combined with a re-hydration aid.The non-cross-linked gelatin will typically be present in an aqueoussolution at from 5% (w/w) to 15% (w/w) and the re-hydration aids will betypically present from 5% to 30% (w/w) based on the weight of gelatin inthe aqueous solution. Preferably, the re-hydration aid comprises PEG atfrom 2.5% to 20% (w/w) based on the weight of the gelatin, PVP at from1.25% to 20% (w/w), and dextran at from 1.25% to 20% (w/w).

The non-cross-linked gelatin together with the re-hydration aid is thencross-linked in any manner suitable to form the hydrogel. For example,polymeric molecules may be cross-linked using bi- or poly-finctionalcross-linking agents which covalently attach to two or more polymermolecules chains. Exemplary bifinctional cross-linking agents includealdehydes, epoxies, succinimides, carbodiimides, maleimides, azides,carbonates, isocyanates, divinyl sulfone, alcohols, amines, imidates,anhydrides, halides, silanes, diazoacetate, aziridines, and the like.Alternatively, cross-linking may be achieved by using oxidizers andother agents, such as periodates, which activate side-chains or moietieson the polymer so that they may react with other side-chains or moietiesto form the cross-linking bonds. An additional method of cross-linkingcomprises exposing the polymers to radiation, such as gamma radiation,to activate the polymer chains to permit cross-linking reactions.Dehydrothermal cross-linking methods may also be suitable. Preferredmethods for cross-linking gelatin molecules are described below.

Exemplary methods for producing cross-linked gelatins are as follows.Gelatin is obtained and suspended in an aqueous solution to form anon-cross-linked hydrogel, typically having a solids content from 1% to70% by weight, usually from 3% to 10% by weight. The gelatin iscross-linked, typically by exposure to either glutaraldehyde (e.g.,0.01% to 0.05% w/w, overnight at 0 C. to 15 C in aqueous buffer), sodiumperiodate (e.g., 0.05 M, held at 0° C. to 15° C. for 48 hours) or1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (“EDC”) (e.g., 0.5% to1.5% w/w overnight at room temperature), or by exposure to about 0.3 to3 megarads of gamma or electron beam radiation. Alternatively, gelatinparticles can be suspended in an alcohol, preferably methyl alcohol orethyl alcohol, at a solids content of 1% to 70% by weight, usually 3% to10% by weight, and cross-linked by exposure to a cross-linking agent,typically glutaraldehyde (e.g., 0.01% to 0.1% w/w, overnight at roomtemperature). In the case of aldehydes, the pH should be held from about6 to 11, preferably from 7 to 10. When cross-linking withglutaraldehyde, the cross-links are formed via Schiff bases which may bestabilized by subsequent reduction, e.g., by treatment with sodiumborohydride. After cross-linking, the resulting granules may be washedin water and optionally rinsed in an alcohol, and dried. The resultingdry powders may then be loaded into the applicators of the presentinvention, as described in more detail hereinafter.

After cross-linking, at least 50% (w/w) of the re-hydration aid will beremoved from the resulting hydrogel. Usually, the re-hydration aid isremoved by filtration of the hydrogel followed by washing of theresulting filter cake. Such filtration/washing steps can be repeated oneor more additional times in order to clean the product to a desiredlevel and to remove at least 50% of the re-hydration aid, preferablyremoving at least 90% (w/w) of the re-hydration aid originally present.

After filtration, the gelatin is dried, typically by drying the finalfilter cake which was produced. The dried filter cake may then be brokenup or ground to produce the cross-linked powder having a particle sizein the desired ranges set forth above.

Kits according to the present invention will comprise a first containerholding the dry cross-linked gelatin powder of the present invention, asdescribed above. The kits will further comprise a second containerholding an aqueous re-hydration medium, typically a saline or otheraqueous solution comprising thrombin which is intended to be mixed withthe gelatin as the gelatin is re-hydrated. The containers can be in anyform, but will preferably be in the form of syringes which permit mixingof the dry gelatin with the re-hydration medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a kit constructed in accordance with the principlesof the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The following examples are offered by way of illustration, not by way oflimitation.

EXAMPLE 1 Preparation of Gelatin Powder

Strips of bovine corium were suspended in a sodium hydroxide solution ofconcentration 1 M to 2 M for 1 hr at room temperature, neutralized withphosphoric acid, and rinsed. The treated strips were then resuspended indeionized water, adjusted to pH 7-8, and heated to 70° C. A homogenizerwas used to further reduce the size of the strips. After 1 hr at 70° C.,the corium was largely solubilized to gelatin. The amount of corium waschosen so that the solids content of the resulting gelatin solution wasapproximately 3-10% (w/w), typically 7-10%. The solution was cast asthin layers onto Teflon® coated metal trays, dried, and ground to formgelatin powder.

EXAMPLE 2 Preparation of “Modified Gelatin Powder”

Re-hydration aids (Table 1) were dissolved in 500 mL of 50° C.de-ionized water and then an amount of bovine derived gelatin powder,prepared as in Example 1, was added to the solution. The finalconcentration of gelatin in solution was chosen to be approximately 8%(w/w, bulk gelatin powder basis), and the total amount of re-hydrationaids in the solution was chosen as in Examples 9-44 (Tables 1 and 2).After the gelatin had dissolved, the solution was poured into Teflon®coated metal trays and dried. The dried gelatin sheet is ground to form“modified gelatin powder”.

Alternatively, strips of bovine corium were suspended in a sodiumhydroxide solution of concentration 1 M to 2 M for 1 hr at roomtemperature, neutralized with phosphoric acid, and rinsed. The treatedstrips were then resuspended in deionized water, adjusted to pH 7-8, andheated to 70° C. A homogenizer was used to further reduce the size ofthe strips. After 1 hr at 70° C., the corium was largely solubilized togelatin. The amount of corium was chosen so that the solids content ofthe resulting gelatin solution was approximately 3-10% (w/w), typically7-10%. Amounts of re-hydration aids were chosen as in Examples 9-44(Tables 1 and 2) and were then added to the gelatin solution, either insolid form or dissolved in a small volume of water. The solution wascast into thin layers onto Teflon® coated metal trays, dried, and groundto form “modified gelatin powder”. Examples of several formulations formodified gelatin are given in Tables 1 and 2.

EXAMPLE 3 Preparation of cross-linked gelatin powder from “modifiedgelatin powder”

600 mL of 0.2 M phosphate buffer (pH 9.2±0.2) was cooled to atemperature below 12° C. 0.32 mL of glutaraldehyde (25%) was added tothe buffer solution and then 20 g of modified gelatin powder was added,resulting in a glutaraldehyde concentration of 4000 ppm (glutaraldehydeto modified gelatin, bulk weight basis). The gelatin was suspended inthe glutaraldehyde solution with a stir bar. The pH of each suspensionswas adjusted to a range of 9.2±0.2 and then maintained at a temperatureof 9 to 12° C. and pH of 9.2±0.2 over 19 hours.

The suspension was filtered and the filter cake was washed withde-ionized water three times by completely covering the filter cake withde-ionized water and then allowing the vacuum to draw the rinse waterthrough the cake. The filter cake was left in the funnel during eachrinse. 0.2 g of NaBH4 was dissolved in 600 mL 25 mM phosphate buffer, pH7.4 0.2, in a beaker. The above filter cake was suspended in the NaBH4solution at room temperature (about 22° C.) for 3 hours, then filteredto remove the liquid.

The filter cake was next suspended in 600 mL of buffer solution at roomtemperature (about 22° C.) for 30 minutes and filtered again. The bufferwas composed of sodium phosphate (dibasic anhydrous and monobasicmonohydrate) and sodium ascorbate. The above procedure was repeatedtwice to ensure that the appropriate ratio of salts to gelatin werepresent to form the desired buffer composition upon reconstitution. Thefilter cake was dried, then ground with a Waring Blender, resulting in“cross-linked gelatin powder”.

This method was also used to prepare cross-linked gelatin powder fromunmodified gelatin powder; that is, gelatin to which no re-hydrationaids were added during its preparation.

EXAMPLE 4 Preparation of Irradiated Product from Cross-Linked Gelatinpowder

About 800 mg (bulk weight) of the cross-linked gelatin powder, preparedas in Example 2, were put into each of several 5 cc syringes. Thesyringes containing powder were sterilized with gamma irradiation atambient temperature.

EXAMPLE 5 Use of Product as a Hemostatic Agent

A syringe of product containing approximately 0.8 g of irradiatedcross-linked gelatin powder was prepared from modified gelatin powder.The modified gelatin powder was prepared as in Example 2. The modifiedgelatin was further cross-linked and irradiated as in Examples 3 and 4.The product was mixed with 4 mL of a saline solution containing about1000 Units of bovine thrombin per milliliter. Mixing was achieved bypassage back and forth between two syringes connected with afemale-female straight-through Luer connector. The powder in the syringewas hydrated as it mixed with the thrombin solution, forming granules ofhydrogel.

A square lesion, approximately 1 cm×1 cm×0.2 cm deep, was created on theliver of a farm-grade pig. The pig had been anticoagulated with heparinso that its activated clotting time (ACT) was three to five times itsbaseline value, and the lesion bled freely prior to treatment. Afterabout 30 seconds from the start of mixing, approximately 2 mL of thehydrated powder was extruded from the syringe onto the lesion and heldin place with compression for two minutes. After compression wasremoved, the treated lesion was observed for bleeding at 3 min, 10 min,and 50 min after application. No bleeding was seen from the treatedlesion at the 3 min and 10 min observation. After the 10 minobservation, the treated lesion was irrigated with saline solution.While excess material was removed, no re-bleeding was observed. At 50min after application, the lesion was observed again and no bleeding wasseen.

EXAMPLE 6 Determination of Re-hydration Rate of a Powder

The “re-hydration rate” of a powder was measured as follows. The powder,packed in a 5 cc syringe, was mixed with a syringe containing a volumeof aqueous solution by passage between the two syringes connected with aLuer fitting for 30 seconds. The volume of aqueous solution was chosento be in excess of what could be expected to be absorbed in 30 seconds.Typically, 0.8 g (bulk weight) of powder was mixed with 3 mL of 0.9%sodium chloride solution. -The-resulting mixture was then immediatelyfiltered to remove any unabsorbed liquid. The wet filtered material wasweighed, then dried in a 120° C. oven for two hours and re-weighed. Thismeasurement gave the total amount of water removed from the wet materialand the weight of the dry powder. The amount of water that had beenabsorbed by the powder was then calculated after a small correction ismade for the residual moisture that had been present in the powderoriginally. The “re-hydration rate” was given as the mass of salinesolution absorbed per gram dry weight of powder in that 30 secondinterval.

In the calculation below, the fraction solids of the bulk powder (“S”)was measured independently by drying the bulk powder at 120° C. for 2 hrand weighing the powder before and after drying. The value of S is givenby the following:$S = \frac{{{weight}\quad{after}\quad{drying}\quad{at}\quad 120{^\circ}\quad{C.}},\quad{2{hr}}}{{weight}\quad{before}\quad{drying}}$

-   -   Re-hydration rate calculation:    -   A: initial weight of the pan and filter paper    -   B: weight of the pan, filter paper and hydrated powder    -   C: weight of the pan, filter paper and sample after drying in        oven    -   S: fraction solids of the bulk powder originally in syringe    -   M: grams of saline absorbed per gram of powder (dry weight)        during mixing (“absorption rate”)        $M = \frac{\left( {B - A} \right) - {\left( {C - A} \right)/S}}{\left( {C - A} \right)}$

EXAMPLE 7 Re-hydration Rate and Physical Property Determination forSeveral batches of powder product

Tables 1 and 2 depict the results of re-hydration rate measurementsperformed on one to for several batches of powder product (Examples9-23). These were made using methods as per Examples 1, 2, 3, and 4.Except for Examples 9 and 17, these were prepared from modified gelatinsthat were made with various proportions of gelatin and the followingre-hydration aids: polyethylene glycol (PEG), average molecular weight1000; polyvinylpyrrolidone (PVP), “k-30” designation, corresponding toan average molecular weight of about 50,000; and dextran, averagemolecular weight 40,000.

It is seen that use of several different combinations of gelatin andre-hydration aids can result in a powder product that absorbs moreaqueous saline solution in 30 seconds per gram of powder than powderproduct made from gelatin to which no re-hydration aids have been added.It is also seen that the combination of gelatin, PEG, PVP and dextran ata bulk weight ratio of 80:10:5:5 in the modified gelatin (Example 10)produces a powder product that absorbs about 33% more saline solutionper gram in 30 seconds than powder product made from unmodified gelatin.

Table 1 also gives values for other physical properties determined forthe powder product lots. “Percent solids” was determined by weighing thepowder before and after drying at 120° C. for two hours to drive offresidual moisture. “DSC peak temperature” refers to the temperature atwhich a peak is exhibited in a thermogram of a differential scanningcalorimetry measurement conducted from 1° C. to 70° C. “Equilibriumswell” was determined by suspending the powder in an excess of salinesolution for at least 18 hr at room temperature. The hydrated powder wasweighed to determine its “equilibrium wet weight” and dried at 120° C.for two hours and re-weighed to determine its “dry weight”. Equilibriumswell is given as${{Equilibrium}\quad{swell}\quad(\%)} = {100{\%\quad \times \frac{{{equilibrium}\quad{wet}\quad{weight}} - {{dry}\quad{weight}}}{{dry}\quad{weight}}}}$

Values for “mean particle size” were measured by light scattering in aCoulter LS particle size analyzer.

From the data presented in Table 1, it appears that the appropriate useof re-hydration aids can change the re-hydration rate of the powderproduct without significantly changing other physical properties.

EXAMPLE 8 Measurement of Polyethylene Glycol, Polyvinylpyrrolidone, andDextran Levels in Modified Gelatin Powder and in Cross-Linked Powder

Approximately 50 mg modified gelatin or 250 mg cross-linked irradiatedpowder product were suspended in 10 mL of deionized water and heated for3 hr at 65° C. The samples were then centrifuged at 15 minutes at 2000rpm. The resulting supernatant was filtered through a 0.45 μm GelmanAcrodisc filter, the first mL being discarded. The resulting sample wasthen assayed by three different high performance liquid chromatography(HPLC) methods to quantitate the polyethylene glycol (PEG),polyvinylpyrrolidone (PVP), and dextran in the sample. For PEG, 100 μLof the sample was injected onto a Waters Ultrahydrogel 120 column,7.8×300 mm, with guard column and prefilter, using deionized water asthe mobile phase. A refractive index detector was used to monitor theeffluent. For PVP, 100 μL of the sample was injected onto a PhenomenexKingsorb C18 5 μm column, 4.6×150 mm, with guard column and prefilter,using a gradient of methanol and aqueous sodium phosphate as the mobilephase. An ultraviolet absorbance detector was used to monitor theeffluent. For dextran, 100 μL of the sample was injected onto a WatersUltrahydrogel Linear column, 7.8×300 mm, with guard column andprefilter, using 0.1 M sodium phosphate, pH 7 and acetonitrile at a90:10 ratio as the mobile phase. A refractive index detector was used tomonitor the effluent. All columns were heated to 40° C. for theanalyses. The limit of quantitation was about 0.1% (w/w sample) for PEGand PVP, 0.2% (w/w sample) for dextran.

Modified gelatin was prepared as per Example 2. The modified gelatin wasanalyzed for PEG, PVP and dextran in the manner described above. Resultsindicated that PEG, PVP, and dextran were present at 16%, 8%, and 3%(w/w bulk) respectively. The modified gelatin was subsequently subjectedto cross-linking, sodium borohydride treatment, and rinsing as perExample 3 to form cross-linked modified gelatin powder. When this powderwas analyzed for PEG, PVP, and dextran by HPLC in the manner describedabove, the content of each of the three re-hydration aids was found tobe below the limit of quantitation.

EXAMPLE 9 Powder Product Made Without Re-hydration Aids

Unmodified gelatin-that is, gelatin to which processing aids were notadded-was prepared from bovine corium strips as in Example 1 andcross-linked as in Example 3. The cross-linked unmodified gelatin wasthen packed into syringes and gamma irradiated as in Example 4. Physicalproperties of the resulting product were measured as in Examples 6 and 7and are given in Table 1.

EXAMPLES 10-23 Powder Product Made With Re-hydration Aids

Batches of modified gelatin were prepared as in Example 2 from gelatinpowder or corium strips and from one, two, or three re-hydration aids.Table 1 gives the proportions of bulk gelatin and re-hydration aidsused. The modified gelatin was then cross-linked as in Example 3. Exceptfor Example 17, the re-hydration aids used were from the following list:polyethylene glycol (PEG) of an average molecular weight of about 1000;polyvinylpyrrolidone (PVP), “k-30” designation, of an average molecularweight of about 50,000; and dextran, of an average molecular weight ofabout 40,000. In Example 17, PEG of an average molecular weight of about400 was used. The cross-linked modified gelatin was then packed intosyringes and gamma irradiated as in Example 4. Physical properties ofthe resulting powder product from each of these preparations weremeasured as in Examples 6 and 7 and are given in Table 1. Data givenwith the formulation for Example 10 is the average and standarddeviation of nine batches prepared according to that formulation.

EXAMPLES 24-44 Powder Product Made With Various Re-Hydration Aids

Batches of modified gelatin were prepared as in Example 2 from gelatinpowder or corium strips and from one of several re-hydration aids. Table2 gives the identity and concentration of re-hydration aid used in eachbatch as a ratio of bulk gelatin weight to re-hydration aid and as apercentage of total bulk solute used to prepare the modified gelatin.The modified gelatin was then cross-linked as in Example 3. Thecross-linked modified gelatin was then packed into syringes and gammairradiated as in Example 4. Physical properties of the resulting powderproduct from each of these preparations were measured as in Examples 6and 7 and are given in Table 2. Data for the Example 9 formulation isprovided in Table 2 for comparison. TABLE 1 Properties of powder productafter Target bulk weight percent in cross-linking and gamma irradiationmodified gelatin (re-hydration aids largely removed) Gelatin PEG DSCMean (bulk MW = PVP Dextran peak temp Equilibrium particle Re- Lotweight) 1000 D MW ˜50000 D MW = 40000 D % solids (° C.) swell (%) size(μm) hydration* No re-hydration aids added Example 9 208-32 100 0 0 088.6 41.3 551 440 2.85 Preferred composition (four-way mixture) Example10 avg of 9 lots 80 10  5 5 87.6 42.1 595 423 3.79 std. deviation 1.01.4 43 65 0.15 of 9 lots Three-way mixtures Example 11 228-69-1 80 10 10 0 88.1 40.8 667 387 3.51 Example 12 228-69-2 80 10  0 10 88.4 40.6670 367 3.14 Example 13 228-78 80 0 10 10 86.7 41.1 632 414 3.20Gelatin-PEG mixtures Example 14 212-39-2 94 6 0 0 86.2 44.4 502 372 2.68Example 15 228-42-3 89 11  0 0 88.6 42.8 594 428 3.16 Example 16228-42-1 80 20  0 0 88.9 42.4 575 312 3.47 Example 17 214-62-1 89  11**0 0 87.1 40.7 599 406 3.11 Gelatin-PVP mixtures Example 18 228-38-3 94 06 0 88.2 42.2 567 399 3.26 Example 19 228-38-2 89 0 11 0 88.3 41.0 605422 3.44 Example 20 228-38-1 80 0 20 0 88.6 42.4 596 401 3.52Gelatin-dextran mixtures Example 21 228-35-3 94 0 0 6 88.1 40.5 631 3953.18 Example 22 228-35-2 89 0 0 11 88.3 41.4 598 345 3.03 Example 23228-35-1 80 0 0 20 88.5 41.9 624 392 3.01*Re-hydration rate defined as grams saline absorbed per gram powderproduct (dry wt) in 30 sec**PEG (MW = 400) used instead of MW = 1000

TABLE 2 Re-hydration aid Conc'n of processing Physical properties aid inDSC MW or bulk modified peak Mean Re- other gelatin gelatin % tempEquilibrium particle hydration Lot type designation wt: aid (bulk wt %)solids (° C.) swell (%) size (μm) rate* Example 9 208-32 no re-hydrationaids 88.6 41.3 551 440 2.85 Example 24 214-11-1 glycerol n/a 4 20%  85.543.4 483 653 2.19 Example 25 214-11-2 glycerol n/a 8 11%  86.4 43.4 529421 2.62 Example 26 214-11-3 glycerol n/a 16 6% 86.5 43.0 543 398 2.35Example 27 214-44-1 dextran 148000 D 4 20%  85.5 nr 634 433 2.62 Example28 214-44-2 dextran 148000 D 8 11%  85.4 nr 607 453 2.57 Example 29214-44-3 dextran 148000 D 16 6% 85.5 nr 603 527 2.33 Example 30 214-44-4dextran 148000 D 32 3% 85.7 nr 531 491 2.37 Example 31 228-35-4 dextran 40000 D 32 3% 84.5 41.4 633 380 2.59 Example 32 214-50-1 PVP k-90 420%  85.3 44.0 612 664 2.41 Example 33 214-50-2 PVP k-90 8 11%  85.644.3 538 581 2.71 Example 34 214-50-3 PVP k-90 16 6% 85.6 44.1 527 5932.78 Example 35 214-50-4 PVP k-90 32 3% 86.1 43.0 597 538 2.76 Example36 214-53-4 PVP k-30 32 3% 87.3 41.1 580 447 2.72 Example 37 214-59-1PEG 400 4 20%  86.7 42.0 595 407 2.18 Example 38 214-66-1 PEG 400 6 14% 86.5 40.8 603 501 2.63 Example 39 212-39-1 PEG 400 16 6% 86.2 43.8 513403 2.11 Example 40 212-39-2 PEG 1000 16 6% 86.2 44.4 502 372 2.68Example 41 214-59-3 PEG 8000 4 20%  87.4 41.5 548 429 2.87 Example 42214-66-3 PEG 8000 6 14%  86.9 41.4 581 426 3.80 Example 43 214-62-3 PEG8000 8 11%  86.8 42.0 631 511 2.78 Example 44 212-39-3 PEG 8000 16 6%86.4 44.6 546 518 2.72nr = not reported*Re-hydration rate defined as grams saline absorbed per gram powderproduct (dry wt) in 30 sec

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

1-25. (canceled)
 26. A method for preparing a substantially drycross-linked gelatin powder, said method comprising: providing anaqueous solution comprising a non-cross-linked gelatin combined with atleast one re-hydration aid; cross-linking the gelatin; removing at least50% (w/w) of the re-hydration aid; and drying the cross-linked gelatinand grinding to produce a powder having a moisture content below 20%(w/w).
 27. A method as in claim 26, wherein the re-hydration aidcomprises at least one material selected from the group consisting ofglycerol, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), anddextran.
 28. A method as in claim 26, wherein the re-hydration aidcomprises at least one material selected from the group consisting ofpolyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and dextran. 29.A method as in claim 28, wherein the re-hydration aid comprises at leasttwo of the materials.
 30. A method as in claim 28, wherein there-hydration aid comprises all the materials.
 31. A method as in claim27, wherein the re-hydration aid is present at a concentration in therange from 5% to 30% by weight based on the weight of gelatin present inthe aqueous solution.
 32. A method as in claim 31, wherein there-hydration aid comprises PEG at from 2.5% to 20% by weight, PVP atfrom 1.25% to 20% by weight, and dextran at from 1.25% to 20% by weight.33. A method as in claim 27 wherein cross-linking comprises adding across-linking agent to the gelatin solution.
 34. A method as in claim33, wherein the cross-linking agent comprises glutaraldehyde.
 35. Amethod as in claim 27, wherein removing at least 50% of the re-hydrationaid comprises filtering the suspension of cross-linked gelatin in asolvent followed by filtration of the gelatin to produce a filter cakeand washing the filter cake to remove the re-hydration aid.
 36. A methodas in claim 35, wherein washing the filter cake removes at least 90%(w/w) of the re-hydration aid originally present in the gelatin.
 37. Amethod as in claim 35, wherein drying comprises drying the filter cakeafter washing, wherein the method further comprises drying the groundfilter cake to produce the dry ground gelatin powder.
 38. A method forpreparing a substantially dry cross-linked gelatin powder, said methodcomprising: providing an aqueous solution comprising a non-cross-linkedgelatin combined with at least one re-hydration aid; cross-linking thegelatin; removing at least 50% (w/w) of the re-hydration aid; and dryingthe cross-linked gelatin and grinding to produce a powder having amoisture content below 20% (w/w) and a re-hydration rate of at least 3gm/gm.
 39. A method as in claim 38, wherein the powder has are-hydration rate of at least 3.5 gm/gm.
 40. A method as in claim 38,wherein the powder has a re-hydration rate of at least 3.75 gm/gm.
 41. Amethod as in claim 38, wherein the powder has a mean particle size inthe range from 150 μm to 750 μm.
 42. A method as in claim 38, whereinthe powder has an equilibrium swell in the range from 400% to 1000%. 43.A method as in claim 38, wherein the powder has a moisture content inthe range from 5% (w/w) to 15% (w/w).
 44. A method as in claim 38,wherein the re-hydration aid is present at below 1% of the powder, byweight.
 45. A method as in claim 38, wherein the re-hydration aid ispresent at below 0.5% of the powder, by weight.
 46. A method as in claim38, wherein the re-hydration aid comprises PEG in the range from 2.5%(w/w) to 20% (w/w) based on the weight of the gelatin.
 47. A method asin claim 38, wherein the re-hydration aid comprises PVP in the rangefrom 1.25% (w/w) to 20% (w/w) based on the weight of the gelatin.
 48. Amethod as in claim 38, wherein the re-hydration aid comprises dextran inthe range from 1.25% (w/w) to 20% (w/w) based on the weight of thegelatin.
 49. A method as in claim 38, wherein the non-cross-linkedgelatin is present in the aqueous solution in the range from 5% (w/w) to15% (w/w).
 50. A method as in claim 38, wherein the rehydration aid ispresent in the aqueous solution in the range from 5% (w/w) to 30% (w/w)based on the weight of the non-cross-linked gelatin in the aqueoussolution.
 51. A method as in claim 38,wherein the rehydration aidcomprises PEG in the range from 2.5% (w/w) to 20% (w/w) based on theweight of the gelatin, PVP in the range from 1.25% (w/w) to 20% (w/w)based on the weight of the gelatin, and dextran in the range from 1.25%(w/w) to 20% (w/w) based on the weight of the gelatin.
 52. A method forpreparing a substantially dry cross-linked gelatin powder, said methodcomprising: providing an aqueous solution comprising a non-cross-linkedgelatin combined with at least one re-hydration aid; cross-linking thegelatin; removing at least 50% (w/w) of the re-hydration aid; and dryingthe cross-linked gelatin and grinding to produce a powder having amoisture content below 20% (w/w) and a re-hydration rate which is atleast 5% higher than a re-hydration rate of a similar powder preparedwithout the re-hydration aid.